This type of a waste heat utilization device for an internal combustion engine has a Rankine cycle provided in its refrigerant channel with an evaporator that heats a refrigerant by using waste heat recovered, for example, from a coolant after cooling a vehicle engine, an expander that expands the refrigerant that has passed through the evaporator and thus produces a rotational drive force, a condenser that condenses the refrigerant that has passed through the expander, and a pump that pumps the refrigerant that has passed through the condenser into the evaporator, in the order named, and a generator that is rotationally driven by the expander and converts the rotational drive force into electric power.
There is a well-known technology that adjusts the amount of the refrigerant circulating through the Rankine circuit by controlling the rotational speed of the expander, and thus improves the recovered power of the expander without reducing the pressure of a high-pressure refrigerant existing on the inlet side of the expander (see, for example, Unexamined Japanese Patent Publication No. 61-150014).
It is known that the expander is driven with maximum efficiency when a pressure ratio of suction pressure Ps of the refrigerant sucked into the expander to discharge pressure Pe of the refrigerant discharged from the expander (Ps/Pe) (so-called expansion ratio), a volume ratio of refrigerant discharge volume Ve to refrigerant suction volume Vs (Ve/Vs), and a specific heat ratio K of the refrigerant passing through the expander are calculated, and the pressure ratio (Ps/Pe) is equal to a value obtained by multiplying the volume ratio (Ve/Vs) by the specific heat ratio K (hereinafter, referred to as a preset pressure ratio), that is, when a relation, the pressure ratio (Ps/Pe)=the preset pressure ratio (Ve/Vs)K, is true (see, for example, Transactions of the Japan Society of Mechanical Engineers. B 54 (506) “Study on Fundamental Performance of Scroll Expander”).
It can be easily analogized from the above-mentioned prior art that the maximum efficiency in the waste heat recovery of the waste heat utilization device is obtained by controlling the rotational speed of the expander so that the relation, the pressure ratio (Ps/Pe)=the preset pressure ratio (Ve/Vs)K, is true, and then adjusting the amount of the refrigerant circulating through the Rankine cycle.
The prior art, however, has never defined specific structure and control for controlling the rotational speed of the expander. Moreover, the maximum efficiency is no more than theoretical efficiency, and the expansion of the refrigerant in the expander is actually a polytropic expansion. The maximum efficiency therefore cannot be obtained even if the foregoing relation is true.